It is known, e.g. from German utility model No. 7,219,776, to drive the shaft of a mixer drum via a planetary-gear train through a gearing with arcuate teeth, the drum shaft being externally journaled in a roller bearing with an outer race forming a toroidal inner guide surface for the rollers; by way of further support, the drum shaft is coupled with a stub shaft of the planetary-gear train through a universal joint which is concentric with the toroidal guide surface of the outer bearing race. This concentric arrangement allows for a certain relative swing between the two shafts, limited mainly by the tendency of the rollers to escape from the bearing if the outer race moves too far from its midposition relative to the inner race.